All Fireworks versions.
The first 5 steps are set to get a file with a black background with white ramdom shapes like camouflage.
Open a 250x250 image and clone it (Ctrl+Shift+D). On the topmost object apply the "Hue/Saturation" live filter with the following setting: Hue: 0 ; Saturation: -100 ; Lighteness: 0.

Figure 1: Hue/Saturation
Add "Noise" live effect with the following setting: Amount = 400; Color: unchecked + Motion Blur with the following setting: Angle: 127; Distance = 5

Figure 2: Add Noise & Motion Blur
Add "Gaussian Blur" live effect with the following setting: Blur radius = 3.3 ; Add "Brightness/Contrast" live effect with the following setting: brightness = -16 ; contrast = 100.

Figure 3: Gaussian Blur & Brightness/Contrast
Add "Blur" live effect.

Figure 4: Spots
When you are happy with the shape of your spots add "Blur" live filter to smooth the border between black and white areas. Then, flatten your object with the Flatten Selection feature. This file will be the mask and the butterfly picture the maskee.
A little snag when you flatten your object. The size of your object goes from 250x250 to 267x267.
Take the mask we have just obtained on top of a butterfly picture. Go to Modify > Mask > Group as Mask in Fireworks menus. Now we have masked our drops, next we going to add some effects to achieve a water look.

Figure 5: Masking
Apply the following Photoshop Live Effects:

Figure 6: Live Effects